54 FOUNDATIONS OF BOTANY 



possessed of intelligence and will. Plants of two different 

 species, both growing in the same soil, usually take from 

 it very various amounts or kinds of mineral matter. For 

 instance, barley plants in flower and red-clover plants in 

 flower contain about the same proportion of mineral mat- 

 ter (left as ashes after burning). But the clover contains 

 5| times as much lime as the barley, and the latter contains 

 about eighteen times as much silica as the clover. This 

 difference must be due to the selective action of the proto- 

 plasm in the absorbing cells of the roots. Traveling by 

 osmotic action from cell to cell, a current of water derived 

 from the root-hairs is forced up through the roots and into 

 the stem, just as the contents of the egg was forced up 

 into the tube shown in Fig. 24. 



66. Root-Pressure. The force with which the upward- 

 flowing current of water presses may be estimated by 

 attaching a mercury gauge to the root of a tree or the 

 stem of a small sapling. This is best done in early spring 

 after the thawing of the ground, but before the leaves 

 have appeared. The experiment may also be performed 

 indoors upon almost any plant with a moderately firm 

 stem, through which the water from the soil rises freely. 

 A dahlia plant or a tomato plant answers well, though the 

 root-pressure from one of these will not be nearly as great 

 as that from a larger shrub or a tree growing out of doors. 

 In Fig. 25 the apparatus is shown attached to the stem of 

 a dahlia. The difference of level of the mercury in the 

 bent tube serves to measure the root-pressure. For every 

 foot of difference in level there must be a pressure of 

 nearly six pounds per square inch on the stump at the 

 base of the tube T. 1 



1 See Handbook, 



